1. Field of the Invention
The present invention relates to a thermal printer and particularly to a technique which makes a positional relation between a platen and a thermal print head adjustable.
2. Discussion of the Background
In a thermal printer, for attaining a high grade printing, the position of abutment between a row of heating elements provided in a thermal print head and a platen is important. That is, it is desirable that the row of heating elements is positioned on a radial line centered on the axis of the platen.
On the other hand, the platen and the thermal print head are used with paper interposed in between. The interposition of paper causes a slight shift of a positional relation between the platen and the row of heating elements. Therefore, the positional relation between the platen and the row of heating elements must be adjusted while assuming a paper-interposed state.
FIGS. 9(A) and 9(B) are schematic diagrams each showing a positional relation between a platen and a row of heating elements provided in a thermal print head. FIG. 9(A) shows a positional relation in case of using thin paper, while FIG. 9(B) shows a positional relation in case of using thick paper. As the thickness of paper l various thicknesses are employable. The position between a platen 2 and a thermal elements row 4 provided in a thermal print head 3 differs between the use of thin paper 1 and the use of thick paper 1. As an example, if a positional relation between the platen. 2 and the -thermal print head 3 is set so that the heating elements row 4 is positioned on a radial line R of the platen 2 while assuming the case where thin paper 1 is interposed between the platen and thermal print head, the position of the heating elements row 4 is displaced from the radial line R in case of using thick paper 1. This is because the thermal print head 3 is displaced in a direction away from the platen 2 by an amount corresponding to the thickness of paper 1. In this case, if the thickness of paper 1 increases, the stiffness of the paper increases as well and the paper 1 itself moves slightly away from the platen 2. Such a phenomenon causes the positional deviation of the heating elements row 4 from the radial line R.
Japanese Patent Laid-Open Publication No. Hei 07 (1995)-125379 describes a technique which offers a measure against dislocation between a platen and a row of heating elements according to paper thickness. According to this technique, a thermal print head is fixed to a bracket with screws so as to permit a slight distance displacement between the bracket and the thermal print head. The direction of the displacement corresponds to a paper conveying direction. Further, plural sets of a combination of a through hole formed in the bracket with an aperture formed in the thermal print head is provided corresponding to the above slight distance displacement and a positioning pin is passed through the through hole and the aperture in a desired set, thereby making a positional adjustment possible between the bracket and the thermal print head. See paragraphs 0006, 0012 and FIG. 1 in Japanese Patent Laid-Open Publication No. Hei 07 (1995)-125379.
According to the technique disclosed in the above laid-open publication, however, when adjusting the position of the thermal print head, it is necessary to perform such operations as loosening the screws used to fix the thermal print head to the brackets, thereafter removing the positioning pin, aligning the through hole in the bracket and the aperture in the thermal print head in the desired set with each other, inserting the positioning pin into the through hole and the aperture, and again tightening the screws. Thus, the operations are complicated. Moreover, the construction involving insertion of the positioning pin into the through hole and the aperture requires a space for -the positioning pin and a work space over an upper surface of the thermal print head. Consequently, the layout of the entire printer is greatly limited.